4. By using double integrals, find the volume bounded by (a) x+y+z=2 in the first octant. (b) (c) 2x+y+z=4, planes x = 0, y = 0 and z = 0. surfaces y = x², y + z = 4 and z = 0. (d) cylinder y²+22=1, plane y=x and yz-plane in the first octant.